Building a basement that will stay dry

The best and easiest moisture control happens at the right stage, when foundations are open and exposed.

Photograph by: Steve Maxwell
, The Ottawa Citizen

OTTAWA — If you’re building a new home, major addition or renovation that involves foundation work, then there are three features required to make your basement reliably dry: a waterproof foundation wall; vertical drainage channels that allow water to move downward, and; horizontal drainage pathways that allow water to flow away from your building.

Many dry basements in Canada don’t have all or even any of these features in place, but why take chances? You never really know for sure how a new basement will behave until it’s all done, but then it’s too late to create meaningful moisture-control features. Multiple layers of safety makes sense, especially when it’s still easy to make them happen.

There’s no shortage of exterior foundation coatings that claim to keep basements dry and it’s essential that something be applied to the outside of basement walls for the most reliably dry operation. Even solid masonry is remarkably porous stuff. Blocks are especially vulnerable to leaks because they’re mostly hollow. Exterior tar brushed onto the outside is the minimum foundation coating required.

Vertical drainage membranes have offered the most effective improvement in basement moisture control over the last 20 years, despite how unlikely they look. These dimpled plastic sheets are secured to the outside of masonry foundation walls, leading from soil level at the top all the way down to drainage pipes at the bottom of the foundation wall. The idea is to keep soil pressure away from the wall surface and this makes all the difference. Without soil pressure, water is free to trickle downward by gravity, without being driven horizontally through the masonry and into your basement. Vertical drainage membranes are amazing.

Directing water away from the underground part of your foundation is the third ingredient required for building reliably dry basements and it’s where things usually fall apart. Perforated pipes (called “drainage tiles”) collect water down near the footings and direct it away to some lower area of land. These pipes need to be nestled in clean, crushed stone, sloped downward slightly and covered with landscape fabric to prevent silt from clogging the passages. It pays to learn the details behind proper foundation drainage installations, then watch and make sure there are no shortcuts on your job.

Any sewer can back up into a basement given the right conditions and, if it happens, it’s almost certainly going to be worse than a flood of ordinary water. This is why a backwater valve is cheap insurance against nasty surprises. The valve is plumbed into the outgoing drainpipe that leaves your home just under the basement floor and it uses a swivelling gate that allows water to flow outward only. Strongly encouraged by a growing number of municipalities, the $100 price tag of a backwater valve is cheap insurance.

If any part of your basement depends on a sump pump to prevent ruined floors and walls, then you need something more than just a regular, plug-in sump pump. Having it operate during power failures is the reason why and a battery-backup sump pump is one option. Designed to operate on a 12-volt car battery, these pumps kick in at a slightly higher water level than your main sump pump. The best models can pump quite a bit of water on a single charge — thousands of gallons in fact.

Your builder only has one easy shot at creating a dry basement and fixing things up later is expensive and wasteful. Though success isn’t complicated, results usually boil down to the kind of diligence delivered by a builder who does a good job even when no one is looking. Finding someone like that is the most important part of getting a reliably dry new basement.

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